1. Field of the Invention
The present invention related to a development device and an image forming apparatus incorporating the development device, and more particularly, to a development device including an agitation mechanism for agitating two-component developer.
2. Description of the Background Art
A related-art image forming apparatus, such as a copier, a facsimile machine, a printer, or a multifunction printer having two or more of copying, printing, scanning, and facsimile functions, forms a toner image on a recording medium (e.g., a sheet) according to image data using an electrophotographic method. In such a method, for example, a charging device charges a surface of an image bearer (e.g., a photoconductor); an optical scanning device emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; the electrostatic latent image is developed with a developer (e.g., a toner) to form a toner image on the photoconductor; a transfer device transfers the toner image formed on the photoconductor onto a sheet; and a fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image onto the sheet. The sheet bearing the fixed toner image is then discharged from the image forming apparatus.
In such an image forming apparatus, either a one-component developer consisting essentially of a toner (e.g., magnetic toner and non-magnetic toner) or a two-component developer including toner particles and carrier particles that carries the toner is used for development. The toner, when agitated and mixed into the carrier, is frictionally charged so as to be electrostatically attracted to the electrostatic latent image formed on the photoconductor. Thus, the toner is consumed during development whereas the carrier is not.
A typical development device, which holds the developer, generally includes a development sleeve, configured to form a magnetic brush of the developer on a surface thereof and to supply the developer to the electrostatic latent image formed on the photoconductor, and an agitation sleeve, configured to supply agitated developer to the developing sleeve. Developer in which the toner has been consumed in the development of the electrostatic latent image formed on the photoconductor is collected and returned to the development device.
Consumption of the toner included in the developer causes a decrease in image density, and therefore fresh toner needs to be supplied to the developer. The fresh toner may be supplied from above a conveyance screw including a screw auger serving as the agitation sleeve, or from an edge of a rotation shaft of the conveyance screw.
The fresh toner is supplied to the developer by controlling a number of rotations of a supply member configured to supply the fresh toner stored in a toner supply unit based on developer concentration detected by a toner concentration sensor or the like. When the fresh toner is poured into the developer in a developer tank, the fresh toner and a carrier are agitated by the conveyance screw provided in the vicinity of the supply member so that the developer is frictionally charged. As a result, the developer having a predetermined or desired charge is supplied to the development sleeve.
In general, the supplied toner is dispersed throughout the developer, and the developer is frictionally charged by being agitated by rotation of the screw auger for a short time until the developer thus prepared is conveyed to the development sleeve. Consequently, the degree of mixing depends in part on the amounts supplied.
Thus, when a larger amount of toner is supplied to the developer, the toner may not be dispersed sufficiently in the developer in the brief time allotted for agitation, and consequently, the toner may not be charged sufficiently when discharged from the developer tank. As a result, weakly charged toner could reach the development sleeve, fouling a surface of the photoconductor and scattering over peripheral components, thereby degrading image quality.
When the screw auger described above is used, only that developer which contacts the screw auger itself is agitated, as is the case when using a stirring paddle. Consequently, the supplied toner may not be sufficiently dispersed in the developer and reliably charged in the developer tank.
One possible method for solving the above-described problem is to provide an interior screw auger and an exterior screw auger positioned respectively inboard and outboard of a rotary shaft, and agitate the developer using multiple flows. However, developer in a gap between the screw augers cannot be agitated, and therefore only providing multiple screw augers, by itself, is not a solution.
In order to improve agitation, it is conceivable to increase the rotation velocity of the rotary member or the number of rotations thereof, or to narrow the gap between the screw augers. However, the screw auger driving system may be damaged due to the increased transfer resistance to the developer when the developer is agitated. Moreover, the toner may be damaged due to increased force of impact on the developer and heat caused by increased friction, increasing stress on the developer.
When the stress on the developer is increased, charging ability may be degraded by pealing away the carrier coating, and the charging ability and the fluidity may be degraded by burying additives in the toner. The degradation of the fluidity causes fluctuation in the charging ability and the transfer rate, thus degrading the image.
Alternative approaches include providing a developer container to agitate and mix developer and fresh toner provided separately from a development portion of the development device, but even with such a configuration, the problems of insufficient mixing of toner and developer as well as stress on the developer particles remain unresolved.